Thank you for your efforts. It works correctly with my Honor 10, but with an adjustment: I absolutely need to disable WiFi+.
However, I am a little surprised: With the same settings and deactivated WiFi+, it does not work with my Honor 8 with Android 8.0.0. Under networks, the display only changes from the mobile network (Telekom) to all networks and not to my private network in which I am located. This means that there is no change between the DNS servers.
Do you have a solution for this?
Greetings Carsten
Edit:
After several attempts and tests, I have to revise my statement from above. Various changes between 2 mobile networks (dual SIM) and my own WiFi showed, that the DNS servers actually change, but the displays under All networks and the Home site are not updated.
I am assuming that the DNS leak test gives me correct results. To put it cautiously, I would claim, that the setting of the Prefer network DNS on the menu does not displayed correctly?